The present invention is based on a sensor for the determination of gas constituents and/or gas concentrations in gas mixtures, particularly CO, NO.sub.x and HC in the exhaust gases of internal-combustion engines. Such a sensor includes a measuring element having a gas sensitive region as well as a pumping cell and pumping electrodes disposed on a solid electrolyte carrier for effecting oxygen transfer. The gas constituents to be determined will hereinafter be called contaminant constituents.
Probes of this type determine the contaminant concentration in exhaust gases by a change in the conductivity of, for example, semiconductive oxides or oxide mixtures. If conductivity is to be used for a measurement of The concentrations of oxidizable or reducible contaminants in exhaust gases, a transverse oxygen sensitivity results. If the oxygen partial pressures are particularly low and also in conjunction with high temperatures, metal oxides, for example SnO.sub.2 or In.sub.2 O.sub.3, experience a reduction of the metal oxide which results in malfunctioning of the sensor.
U.S. Pat. No. 4,158,166 discloses a method of measuring combustible constituents in a fuel-gas atmosphere wherein an electrochemical measuring cell and a pumping cell are provided. The measuring cell sets the electrical pumping potential to such a level that sufficient oxygen is pumped into the interior chamber to cause combustion of the combustible constituents. The amount of oxygen pumped is precisely the amount required to always produce a stoichiometric mixture at the measuring cell. The pumping current is utilized as a measure for the percentage of combustible constituents.
If oxygen is pumped toward the interior pumping electrode, the diffusion inhibiting effect of the protective layer causes an oxygen partial pressure which is higher than that of the exhaust gas to develop in the sensitive region. However, part of the oxygen diffuses through the layers and through the protective layer disposed between the sensitive region and the exhaust gas, and acting as a diffusion barrier, and reaches the oxygen-poor exhaust gas. The magnitude of the oxygen partial pressure is thus determined by the magnitude of the O.sub.2 pumping current, the porosity of the individual layers, and the cross-sectional area determinative for the O.sub.2 diffusion current. It is of advantage to have an excess oxygen or the highest possible oxygen partial pressure in the sensitive region compared to the oxygen partial pressure in the exhaust gas to thus eliminate as much as possible the dependency of the sensitive region on oxygen partial pressure fluctuations in the exhaust gas. This can be realized, on the one hand, by increasing the O.sub.2 pumping current. However, any arbitrary increase of the pumping current is restricted to physical and electrochemical limits. On the other hand, the increase in oxygen partial pressure in the sensitive layer can be realized by a thicker and/or denser protective layer. However, this reduces the sensitivity of the sensitive region for the contaminant constituents to be measured in the exhaust gas.